Many wireless devices include antennas that are printed or mounted on Printed Circuit Boards (PCBs) with other circuits. During operation, currents in an antenna may couple with currents in wires on the PCB. Coupling is a phenomenon that is known to designers of electromagnetic devices, and it involves both capacitive and inductive effects and includes the transfer of electromagnetic energy between one current and the other current.
Coupling is illustrated in FIGS. 8A-C. The greatest amount of coupling occurs with parallel currents, as in FIGS. 8A and 8C. If the currents are in opposite directions, the current generally cancel, at least partially, if the spacing between the conductors is within approximately one-sixteenth of a wavelength. On the other hand, currents in the same direction will generally add when spaced within approximately one-sixteenth of a wavelength. The least amount of coupling generally occurs with perpendicular currents, as in FIG. 8B.
As explained above, when two currents are coupled, the two affect each other additively or subtractively, and a change in one will usually cause a change in the other. Thus, when a radiating structure has a current that is coupled to a second current, a change in the second current can affect the radiation performance of the radiating structure. In other cases, especially when a PCB includes materials that readily absorb Radio Frequency (RF) energy and turn it into heat, such coupling can further lower total system performance. Coupling is generally seen by designers as a problem or something to be worked around. However, it is difficult to eliminate all coupling.